It has long been known in the art to form ionically conducting polymer electrolyte membranes and gels from organic polymers containing ionic pendant groups. Well-known so-called ionomer membranes in widespread commercial use are Nafion® perfluoroionomer membranes available from E. I. du Pont de Nemours and Company, Wilmington, Del. Nafion® is formed by copolymerizing tetrafluoroethylene (TFE) with perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride), as disclosed in U.S. Pat. No. 3,282,875. Other well-known perfluoroionomer membranes are composed of copolymers of TFE with perfluoro (3-oxa-4-pentene sulfonyl fluoride), as disclosed in U.S. Pat. No. 4,358,545. The copolymers so formed are converted to the ionomeric form by hydrolysis, typically by exposure to an appropriate aqueous base, as disclosed in U.S. Pat. No. 3,282,875. Lithium, sodium and potassium are all well known in the art as suitable cations for the above cited ionomers.
It is known that membrane conductivity can be improved by reducing the equivalent weight of the polymer comprising the membrane. However, reducing equivalent weight to obtain high conductivity gives rise to problems with poor mechanical properties in proton conductive membranes. One approach to improve mechanical properties is to prepare crosslinked ionomers. Crosslinked terpolymers of TFE, perfluorovinyl ethers containing sulfonyl fluoride, and fluorinated dienes are disclosed in European patent EP 1172382. Various crosslinkers are disclosed in European patent EP 1167400 and U.S. Pat. Nos. 6,214,955 and 6,255,543 disclose polymers containing cyclic repeating units of selected partially fluorinated monomers.
What is needed, are new crosslinked polymers that can be formed into conductive proton conductive membranes with good mechanical properties.